Streaming tape drives are used in computer systems to store digital data on magnetic tape. Data is recorded on the magnetic tape in a serial fashion along a plurality of substantially parallel data tracks which run the length of the tape. When one data track has been fully recorded, the direction of the tape is reversed, and another data track is recorded in the opposite direction. This bidirectional movement of the tape over the magnetic head obviates the need for rewinding the tape after each track is recorded. This back and forth recording of data along a plurality of data tracks one at a time is known as "serpentine" recording.
Streaming tape drives are advantageous in that the serpentine method of recording allows for the storage of a relatively large amount of data on the magnetic tape cartridges that are used in streaming drives. A single tape cartridge recorded in serpentine fashion may hold hundreds of megabytes of data. Another advantage resulting from the use of streaming tape drives is that the tape cartridges produced are portable and can be easily transferred from one computer to another. In contrast, floppy disks, or diskettes, which are also easily transportable, typically have capacities on the order of hundreds of kilobytes. Hard disks used in computers have a relatively large storage capacity, but cannot be easily transported from one computer to another.
While streaming tape drives produce large capacity tape cartridges, they typically have poor performance in the retrieval of selected portions of data which results from the fact that the data stored on a serpentine tape cartridge is essentially a single, long data track of serially recorded data. In the past, in order to retrieve a selected portion of data, streaming tape drives have searched the entire data string for the presence of the selected portion of data. For example, if the magnetic read/write head of the tape drive happened to be on the first data track, or track 0, and a selected portion of data was recorded on the seventh data track, or track 6, the tape drive would search each of the intervening data tracks in their entirety in sequential fashion to locate the selected data. Thus, the drive would search track 0, and then track 1, and then track 2, and so on, until it found the selected data on track 6.
Because of this time-consuming search process, streaming tape drives have typically been used only as backup storage devices. One such typical application is to store the entire contents of a hard disk onto a streaming tape cartridge at the end of the day. This application takes advantage of the large storage capacity of the tape cartridges and avoids the disadvantage of the long data retrieval time since data is only being written to the tape and not read from the tape. In the somewhat unusual circumstance of a hard disk failure, the entire data content of the hard disk that is lost may then be retrieved from the tape cartridge.
However, the use of streaming tape drives only as backup storage devices does not take full advantage of the large storage capacities of magnetic tape recorded in serpentine fashion and the portable nature of magnetic tape cartridges.